Compensated amplifier having pole zero tracking

Abstract

A circuit for producing a compensated output signal includes a first and second stage of amplification (18) and (20) and a compensation circuit (16). The first stage of amplification (18) has a signal input, a signal output and a control input. The second stage of amplification (20) also has a signal input, a signal output and a control input with the signal input of the second stage (20) connected to the signal output of the first stage (18). A passive feedback network comprised of a series connected resistor (R F ) and capacitor (C F ) is connected between the signal input and the signal output of the second stage (20). The combination of the passive feedback network, the first stage (18) and the second stage (20) provide a gain and frequency response that is defined by three poles and one zero. The zero overlaps one of the poles thereby providing an extended frequency response. The passive feedback network varies the frequency response in response to manufacturing process variations. A compensation network (16) is connected to the control inputs of the first and second stages (18) and (20). The output of the compensation network (16) varies in response to manufacturing process variations and compensates the variation of the frequency response due to the feedback network, thereby providing an amplifier having a frequency response independent of manufacturing process variations.

We claim: 
     
       1. A circuit for producing a compensated output signal comprising: first amplifying means having an input and an output;   second amplifying means having an input and an output, the input of said second amplifying means connected to the output of said first amplifying means;   passive feedback means comprising a first resistive element connected between the input and the output of said second amplifying means for determining the frequency response of the combination of said first and second amplifying means, said passive feedback means varying the frequency response in response to manufacturing process variations;   compensating means for varying the frequency response of said first and second amplifying means in response to manufacturing process variations, said compensating means comprising a second resistive element; and   said passive feedback means and said compensating means manufactured in a common process wherein said compensating means counteracts frequency response variations of said first and second amplifying means resulting from said passive feedback means.   
     
     
       2. The circuit of claim 1 wherein said first amplifying means comprises a differential amplifier. 
     
     
       3. The circuit of claim 1 wherein said second amplifying means comprises a single stage common source amplifier. 
     
     
       4. The circuit of claim 1 wherein said passive feedback means further comprises a capacitor. 
     
     
       5. The circuit of claim 1 wherein said compensating means further comprises: a current source having a current that varies in response to manufacturing variations of said second resistive element;   a first current control means for proportionally reflecting the output of said current source to said first amplifying means; and   a second current control means for proportionally reflecting the output of said current source to said second amplifying means.   
     
     
       6. A tracking amplifier, comprising: a current reference circuit which includes a first resistor for determining the current flow through said current reference circuit;   a first amplifier stage for receiving an input signal, said first amplifier stage including a first current source which passes the current through said first amplifier stage, the control terminal for said first current source connected to said current reference circuit;   a second amplifier stage having an input thereof connected to the output of said first stage and generating an output signal for said tracking amplifier, said second amplifier stage including a second current source which passes the current through said second amplifier stage, the control terminal for said second current source connected to said current reference circuit, said second stage including a feedback circuit comprising a serially connected capacitor and second resistor, said first and second resistor fabricated in the same processing operation wherein parameter variations in said first resistor track with the parameter variations in said second resistor.   
     
     
       7. A circuit for producing a compensated output signal comprising: first amplifying means having a signal input, a signal output and a control input;   second amplifying means having a signal input, a signal output and a control input;   the signal input of said second amplifying means connected to the signal output of said first amplifying means;   passive feedback means comprising a first resistive element connected between the signal input and the signal output of said second amplifying means for determining the frequency response of said first and second amplifying means, said passive feedback means varying the frequency response as a result of manufacturing process variations;   compensating means connected to the control input of said first and second amplifying means for varying the frequency response of said first and second amplifying means in response to manufacturing process variations, said compensating means comprising a second resistive element; and   said passive feedback means and said compensating means manufactured in a common process wherein said compensating means counteracts frequency response variations of said first and second amplifying means resulting from said passive feedback means.   
     
     
       8. The circuit of claim 7 wherein said first amplifying means comprises: a differential amplifier coupled to the signal input and the signal output of said first amplifier means; and   a current source in series with the source of said differential amplifier, said current source coupled to the control input of said first amplifier means for varying the current therethrough.   
     
     
       9. The circuit of claim 7 wherein said second amplifying means comprises: a single stage common source amplifier coupled to the signal input and the signal output of said second amplifier means; and   a current source in series with the drain of said common source amplifier, said current source coupled to the control input of said second amplifier means for varying the current therethrough.   
     
     
       10. The circuit of claim 7 wherein said passive feedback means comprises a serially connected resistor and capacitor. 
     
     
       11. The circuit of claim 7 wherein said compensating means further comprises a current source having a current that varies in response to manufacturing variations of said second resistive element, the output of said current source providing a control signal to the control input of said first and second amplifying means. 
     
     
       12. The circuit of claim 11 wherein said second resistive element determines said current of said current source. 
     
     
       13. A circuit for producing a compensated output signal comprising: a first stage of amplification having a signal input, a signal output and a control input;   a second stage of amplification having a signal input, a signal output and a control input, the signal input of said second stage of amplification connected to the signal output of said first stage of amplification;   a passive feedback network comprised of a first resistive element connected between the signal input and the signal output of said second stage of amplification, the combination of said passive feedback network, said first stage of amplification and said second stage of amplification having a gain and a frequency response defined by three poles and one zero, the zero overlapping one of the poles, said passive feedback network varying the frequency response in response to manufacturing process variations; and   a compensation network comprised of a second resistive element connected to the control inputs of said first and second amplification stages, the output of said compensation network varying in response to manufacturing process variations, said passive feedback network and said compensation network manufactured in a common process wherein the processing variance of said compensation means varies the frequency response to counteract the variation in frequency response resulting from the processing variance of said passive feedback network, thereby providing a circuit having a frequency response substantially independent of manufacturing process variations.   
     
     
       14. The circuit of claim 13 wherein said first stage of amplification comprises: a differential amplifier coupled to the signal input and the signal output of said first stage of amplification; and   a variable current source in the source of said differential amplifier coupled to the control input of said first stage of amplification for varying the current thereof.   
     
     
       15. The circuit of claim 13 wherein said second stage of amplification comprises; a single stage common source amplifier coupled to the signal input and the signal output of said second stage of amplification; and   a variable current source in the source of said amplifier coupled to the control input of said second stage of amplification for varying the current thereof.   
     
     
       16. The circuit of claim 13 wherein said passive feedback network further comprises a capacitor. 
     
     
       17. The circuit of claim 13 wherein said compensation network further comprises a current source having a current that varies in response to manufacturing process variations of said second resistive element, the output of said current source providing a control signal to the control input of said first and second amplification stages. 
     
     
       18. The circuit of claim 17 wherein said second resistive element determines said current of said current source.